The present invention relates to an electronically controllable brake booster. In particular, the present invention relates to an electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from each other by a movable wall, a control valve arrangement which can be actuated by means of an electromagnetic actuation means, and by means of which a pressure difference between the pressure chamber and the vacuum chamber can be adjusted, with the control valve arrangement, as a function of a current flowing through the electromagnetic actuation means, assuming a holding position in which the current ranges between a higher value and a lower value without the control valve arrangement leaving its holding position, a first pressure changing position in which the current is higher than the higher value, and a second pressure changing position in which the current is lower than the low value.
From DE 195 27 493 A1 an electromagnetic actuation means is known which comprises a solenoid coil which can be subjected to a control current and an armature which is associated with the solenoid coil and which is adapted to perform movements which are a function of a control current flowing through the solenoid coil and a spring arrangement which biases the armature in the opposite direction. Therein, a holding position is defined as a manipulated variable, which the armature assumes at a holding current through the solenoid coil. This holding current flowing through the solenoid coil can be changed to a higher value or to a lower value without the armature leaving the holding position.
In addition, both the higher and the lower current value are dimensioned in such a manner that interfering influences on the magnetic and spring forces actuating the armature do not bring the armature into an actuated position which differs from the holding position.
For this purpose, the higher current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined; the control current is increased by a predetermined current step in a stepwise manner and output as a manipulated variable to the solenoid coil of the electromagnetic actuation means, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the second actuated position.
The lower current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined; the control current is decreased by a predetermined current step in a stepwise manner, and output as a manipulated variable to the solenoid coil of the electromagnetic actuation means, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the first actuated position.
From this document is it also known that the control valve arrangement can reliably be brought into the holding position if the arithmetic mean value of the currents is selected for the holding current.
However, the "Learning of the working points" covered in DE 195 27 493 A1 is limited in that the decisive currents for the lower limit and the upper limit must be learned and stored at each commencement of a trip upon switching on the ignition or in periodic time intervals during driving. Upon controlling the control valve arrangement, the currents which have been learned in this manner are used for specifying the first manipulated variable in order to achieve an adequate control behaviour.
In addition, the pressure difference acting upon the movable wall of the brake booster, which can be adjusted to different values depending upon the desired control, is not considered quantitatively.
The pressure difference adjusted at the movable wall is also acting immediately upon the valve body, the valve seat, and the valve element, which also leads to a shift of the currents for the lower limit and for the upper limit.